The evaluation of an electrocardiogram includes the determination of the maximum vector in the electrical QRS quantity in the frontal plane and is described by such terms as "left position", "steep position", etc. In addition, the direction of the repolarization vector, i.e., the behavior of the electrically detectable T-wave, is considered, particularly with respect to the direction of the QRS-vector (concordant or discordant behavior of the T-wave). An example is the behavior of the R and T components in the course of a myocardial infarction and the recovery therefrom. However, these evaluations are only possible in a roughly qualitative manner and, in addition, they are often considerably falsified by projection-caused errors because they are based only on the projection of these vectors on a plane but fail to take account of the divergence of the vectors or of components thereof perpendicular to the plane.
The following conclusions can be drawn on the basis of the ambiguity of the projections known from the representing geometry. All ECG derivations are projections of the true angle in space onto a plane. It is standard practice to consider a concordant T as normal and a discordant T as abnormal. Both can be correct, but as a result of the ambiguity of the projections, both can also be incorrect. Experience has shown that it is very difficult to evaluate a T-wave as to whether it is normal or pathological.
Thus, it is not possible to make a conclusive evaluation without knowing the behavior of the two associated vectors in space. It is therefore necessary to use a three-dimensional or orthogonal derivation system to obtain more accurate information and for quantitatively determining changes to the maximum vectors of QRS and T.
The presently recognized method for the construction of orthogonal derivations (Paul Lichtlen, Klinische Vektor-Elektrokardiographie, published by Springer, Berlin, Heidelberg, New York) consists of measuring individual local electrical voltages on the surface of a thorax model, the voltages being produced by an internally introduced artificial electric dipole. Accompanied by the upstream connection of a resistance network, these voltages are combined to form three derivations which correspond to the projection of the artificial electric dipole on the frontal, sagittal and horizontal planes of the thorax model. This takes place under the idealized assumption that the electrical field is, as a simplification, a dipole with a fixed neutral point.
This constitutes the SVEC III system of Schmitt and Simonson (1955), that of Frank (1956), and that of McFee and Parungao (1961). The reproducibility in all of the systems is good and they are recognized as being equivalent to each other. However, even the authors have admitted that all three systems give precise orthogonal derivations of only a model and the orthogonality can not be strictly obtained on humans due to individual variations in body dimensions and individual heterogeneous electrical conductivity characteristics in the tissues surrounding the heart. It is therefore not surprising that measurements on the same test subject with each of the three systems can easily lead to diverging results, with regard to vector direction (azimuth and elevation) as well as to the vector length i.e., the magnitude (Schmitt 1956, Tuna 1980).
A further disadvantage of these three systems is the complicated derivation technology with 14, 7 or 9 electrodes. As a result, the method is complicated for clinical use and is also very fault-prone so that it has not, as yet, become widely used in a routine manner in clinics.
The search for a simpler derivation system has revealed that it should be theoretically possible to construct orthogonal derivations from only four points on the thorax. This idea is not new and, in 1936, Schellong developed a derivation system with four electrodes. Using the term "vector diagram", the employed three derivations at right angles to one another, namely a horizontal from two points, the infraclavicular left (point zero) and right (point one), a vertical from point zero downwards to the thorax, approximately to point V of Wilson (referred to by Schellong as point three) and a sagittal from point zero to the dorsal (point two). He considered these three derivations as projections of the dipole and, in each case, linked two of these to form a loop which he made visible with a Braun tube. However, this technically simple method proved to be inaccurate and there were distortions of the loop. Duchosal and Sulzer (1949) used the same cubic system but, to avoid these distortions, chose the zero point (the origin of the system of coordinates of three axes) as far away as possible from the heart, namely in the back of the body to the rear and to the right. However, this system was also not adopted, although the coincidence with the biophysical derivation system SVEC III of Frank and McFee was not all bad (cf. Schmitt, 1956). The lack of precision of all cubic systems is due, inter alia, to the premise that each bipolar derivation represents the direct projection of the dipole moment of the heart. This can not be so, because each derivation is merely a potential difference measurement, i.e., a non-directional or scalar quantity, whereas the dipole moment, apart from its magnitude, also has a clearly defined direction, i.e., a vector character (Irnich, 1976).